Ferroelectricity in complex chalcogenides M′M″P₂X₆(M′, M″ - Sn, Pb, Cu, In, Cr; X - S, Se)

For complex thio- and selenophosphides the character of phase transitions and the vibration spectra of the crystal lattices are compared. The splitting of internal vibrations of (P2X6)4- anions reflects the anisotropy of crystal fields. The substitution of the chalcogen atoms in anion sublattice or the metal atoms in cation sublattice changes the mechanism of the phase transitions essentially by variation of the frustration in competing interatomic forces, in first case, and by the influence on the balance of local forces in second one.